Underwater speed?

[AvionicsMaster1]

Holy macaroni! That's bigger than a few compact cars I've rented.

I'm more familiar with aircraft and they do reference airspeed to altitude(pressure density) but mostly to find Mach. They reference airspeed to temperature to find true airspeed. Temp would be easier than a differential pressure to sense I think. Would water density or (there has to be a term for the column of water above the item) induce much of an error? I guess I assumed a local pond or small lake not the ocean.

What depth is absolute maximum? Please don't tell me the bottom of the lake. (It's an old submariners joke.)

[Ioannis]

I wonder, how do you control the sub when it is under water line?

Ioannis